Grow Q&A: Algae on Rockwool

A friend of mine started some clones with rockwool and noticed some mold developing on top of the rockwool after a few days. How can this be prevented? – Rucko

Dear Rucko,
One of the advantages to using rockwool as a growing medium is that it stays wet, yet allows oxygen to reach the roots. This can also cause mold and algae to build up on the surface of your cubes, and they in turn begin to compete with your plants for your nutrient solution. Luckily, They need light to live, so if you simply cover the tops of your rockwool cubes with an opaque surface, you’ll eliminate most algae and mold growth on your medium. Some companies make products specifically for this purpose. The one that comes to mind is Cubecap (cubecap.ca). Covering your cubes also has the added benefit of discouraging annoying fungus gnats as well.